22 Sep 2017

Red kangaroos are
the Conor McGregor of kangaroos, and it’s not because of their hair colour. They
are tough, really tough. Unlike grey kangaroos, which typically seek shade in woodlands
and mostly depend on human-built water holes, red kangaroos don’t shy away from
living in the driest, hottest deserts.

To cool down
their bodies and avoid overheating (and death), kangaroos may pant, sweat and
even lick themselves. But all these strategies to lower body temperature come
with a price: they use up body water. And when you’re living in a place where
water is a rare commodity, licking yourself profusely might not always be the best
idea… So how do red
kangaroos manage to avoid overheating and save body water at the same time?

To answer this
question, Dale Nelson, Gavin Prideaux, and Natalie Warburton from Flinders and Murdoch
Universities decided to take a close look at the red kangaroo’s nose. Yes, the nose.

Red Kangaroo (Macropus rufus).

Mammals have
complex noses with narrow, curled spongy bones that work as an air conditioning
system. These so-called turbinate bones
are lined with thin blood vessels that make a temperature gradient along the nasal
cavity—from cooler near the exterior to warmer internally. As we inhale, the incoming
air is quickly warmed as it travels down the nasal passages, and when we
exhale, warm air coming from the lungs is cooled, which saves body heat. Turbinate bones have another function though, and this is where red kangaroos come
back into the story.

In the late
1970s, a few research teams noticed that desert mammals have extravagantly long
turbinate bones. Camels, for example, have very long, convoluted turbinate
bones that swirl round and round like a corkscrew. This scrolled shape
increases the nasal surface area to about 1000 cm2, which is over
six times the nasal surface area of humans. But what’s the advantage of having
such extreme noses in the desert?

Scientists back
then suspected it must have something to do with saving body water, and they
were right. It turns out that in these animals the water vapour in exhaled air
condenses as it contacts the cooler nasal surface, turning into liquid water. For example, giraffes may save
up to 3 liters of water a day by condensation in the nose. But as impressive as this may sound, how this water
is reabsorbed into the body has remained a mystery for over three decades.

Now, Nelson, Warburton,and Prideaux add the first piece to this puzzle
in a new study published in the Journal
of Zoology.

During work on fossil
kangaroosat Flinders University, the team started wondering how the many
shapes of noses in different species of kangaroos and wallabies might be
related to their environment and behaviour. They were especially intrigued by the
bulging noses of red kangaroos.

“Red kangaroos are the most
adapted to the very hot, arid conditions of the Australian outback”, says
Warburton. “Previous studies had described some aspects of nasal morphology in
kangaroos, but we still didn’t really understand how these related to the
biology of the animals in the wild.”

They set off to examine the internal
bones and tissues of these animals expecting to find very long, coiled
turbinate bones, like in other desert mammals, but they discovered something that
“has never been found before”, Warburton says.

Digital images from
CT-scans, the same technology used in hospitals to image internal body
structures, revealed a pocket of bone within the floor of the nasal cavity. This small hole in the bone was unusual,
so the researchers used histological techniques to look carefully at the tissues
lining it. To their surprise, they found that the bone pocket was filled with
lymphatic vessels.

Lymph vessels are responsible
for returning fluid from tissues into the circulating blood, so this pocket
could be used to reabsorb the water condensed in the nose into the body.

“The condensation of water
vapour from air as animals breathe out is known to […] conserve water in arid
environments, but this is the first time that a possible mechanism for the
reabsorption of that condensed water has been found in the nose of any mammal”,
says Warburton.

Kidneys are the main site of
water reabsorption in the body, and this is why in hot days we need to visit
the WC less often. Desert mammals including kangaroos have special kidneys that
produce very concentrated urine, which helps to save body water.

Nelson and colleagues may have
discovered a new mechanism of water reabsorption in the nose that helps explain
how desert mammals cope with the harsh conditions of their environment,
but “further physiological testing is necessary to see if this is what is
really going on”, Warburton claims.

In the future the team also plans
to look at fossils of kangaroos to try and understand how extinct species
adapted to changes in their environment.“Through understanding how
animals interacted with the environment in the past, we are able to better
predict how they might adapt to environmental changes in the future”, Warburton
concludes.

18 Jul 2017

About 570 million
years ago, large, frond-like creatures suddenly invaded the ocean floors. For
over a billion years, the Earth’s oceans were filled with bacteria and
microscopic algae, but during the Ediacaran period, from 635 to 541 million
years ago, larger multicellular organisms began crowding the seas.

Fossil imprints from
the Ediacaran derive from soft-bodied organisms resembling modern-day sea
anemones (Cyclomedusa), annelid worms
(Dickinsonia) and sea pens (rangeomorphs
such as Charnia). Among these bizarre
creatures, the rangeomorphs are the most abundant in the fossil record—and also
some of the largest.

Artist impression of rengeomorphs (credit: Jennifer Hoyal Cuthill)

Rangeomorphs were unlike
any creature on Earth today. Some were as small as a coin, while others could
grow up to 2 meters high. They looked like ferns, with branches spreading out
from a central stem, but they likely fed by filtering nutrients from the water,
similar to corals. Because rangeomorphs were so different from any known life
form, paleontologists still don’t agree whether they were primitive animals related
to soft corals, some sort of weird fungus or even a new (now extinct) kingdom
of life, the Vendobiota.

These ocean dwellers eventually
disappeared after the Cambrian explosion, some 541 million years ago, when
fast-moving predators emerged (and probably ate them).

Changes in ocean chemistry

Based on the chemical signature
of ancient seawater left on rocks, geochemists think there was a sharp rise in ocean
oxygen levels soon after the end of the Gaskiers glaciation, about 580 million
years ago. These changes in the ocean chemistry could explain the appearance of
larger and more complex marine organisms—more food, bigger bodies. However, even
though this may seem quite obvious, it’s actually quite difficult to
demonstrate.

Jennifer Hoyal Cuthill
and Simon Conway Morris, from the University of Cambridge (UK) and Tokyo
Institute of Technology (Japan), used an original approach to tackle this problem.

“We wanted to see
whether the increase in body size could point to a rise in oxygen, since the
type of growth can tells us whether the animals have nutrients available or not”,
says Hoyal Cuthill.

They suspected that
Ediacaran organisms were large because they had a ‘nutrient-dependent’ type of growth,
rather than an evolutionarily new genetic makeup.

‘Seeing’ extinct creatures grow

Many organisms can’t
grow beyond a certain size, regardless of how much they eat. Humans for
example, will (unfortunately) just get fatter, not taller, because they are
genetically programmed to reach a specific maximum height. But for some
organisms nutrient availability can affect body size. This type of nutrient-dependent growth is quite
common in invertebrates and plants. Some plants will grow almost indefinitely,
as long as there are nutrients (and light) available in the environment.

But how do you measure
growth in organisms that lived nearly 600 million years ago?

This is where
rangeomorph fossils come in handy.

Hoyal Cuthill and Conway
Morris had previously worked with several rangeomorph specimens to study the
unusual body plan of these animals. During this research it dawned on them that
the rangeomorphs’ complex fractal branching shape, with larger older branches
at the bottom and smaller younger branches on top, was the key for testing the nutrient-dependent
growth hypothesis.

“It’s like looking
back at your childhood photographs and comparing your height through your old
photos up to the present day”, says Hoyal Cuthill. “We were inferring the
history of growth of a rangeomorph by looking at parts of the structure of
different ages”.

The researchers could
basically “see” in a single fossil specimen how the animals were growing during
their lifetime, by comparing the relative size and shape of younger and older
branches.

A unique rangeomorph fossil

Fossil of Charnia (Jennifer Hoyal Cuthill)

The new study focuses
on an exquisitely preserved specimen of Avalofractus
abaculus, one of the last fossils removed from the Trepassey Formation, in
Newfoundland (Canada), before strict restrictions were imposed to protect the
site (currently called Mistaken Point Ecological Reserve). Hoyal Cuthill
obtained a high-resolution cast from the Royal Ontario Museum and scanned it by
CT- microtomography, a technique which uses x-rays to make detailed digital 3D
reconstructions.

Two other specimens (Charnia masoni and an undescribed specimen
from the South Australian Museum) were also analysed based on digital
photographs.

Mathematical and
computer models comparing the surface area and the volume of younger and older
branches showed that growth gradually slowed down as rangeomorphs got bigger,
which is exactly what happens in modern organisms with nutrient-dependent
growth.

“… You’re getting less nutrients as you
get larger, so you cannot sustain the same rate of growth, and it slows down”, Hoyal
Cuthill explains.

But there was more. Nutrient
availability can also affect body shape, which is technically called
ecophenotypic plasticity. Hoyal Cuthill and Conway Morris also found that
rangeomorphs could rapidly change shape to access higher levels of oxygen in
the seawater above them, by growing into a long, tapered shape.

Nutrient-dependent
growth provides a mechanism to explain why changes in ocean chemistry caused
the appearance of these large organisms in the Ediacaran, some 30 million years
before the Cambrian explosion.

Hoyal Cuthill next
wants to investigate whether rangeomorphs really are animals, and to which modern
groups are they related to.

“Rangeomorphs are quite mysterious and
were only relatively recently discovered and identified as Precambrian
organisms”, she says. “This is an exciting time and many researchers are
looking at the biota of the Ediacaran and finding new fascinating things”.Reference: Hoyal Cuthill, Jennifer F., and Simon Conway Morris. "Nutrient-dependent growth underpinned the Ediacaran transition to large body size." Nature Ecology and Evolution (2017). DOI:10.1038/s41559-017-0222-7This article was published originally as a guest post in the PLOS Paleo Community blog with the title "Why Precambrian life got so big" on the 18-07-2017. You can read it here.